Method and apparatus for encoding video, and method and apparatus for decoding video

ABSTRACT

Disclosed are method and apparatus for encoding and decoding an image which divides a current picture into blocks with various sizes and varies a number of intra prediction modes according to the sizes of the divided blocks.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of Korean Patent Application No.10-2009-0075854, filed on Aug. 17, 2009, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein in itsentirety by reference.

BACKGROUND

1. Field

The exemplary embodiments relate to encoding and decoding a video, andmore particularly, to a method and apparatus for encoding and decoding avideo which performs intra prediction by selecting an intra predictionmode according to a size of an intra-predicted data unit.

2. Description of the Related Art

According to an image compression standard, such as moving pictureexpert group (MPEG)-1, MPEG-2, MPEG-4, or H.264/MPEG-4 advanced videocoding (AVC), a picture is split into macroblocks for video encoding.After each of the macroblocks is encoded in any of inter prediction andintra prediction encoding modes, an appropriate encoding mode isselected according to a bit rate required for encoding the macroblockand an allowable distortion between the original macroblock and thedecoded macroblock. Then the macroblock is encoded in the selectedencoding mode.

As hardware for reproducing and storing high resolution or high qualityvideo content is being developed and supplied, a need for a video codecfor effectively encoding or decoding the high resolution or high qualityvideo content is increasing. In a conventional video codec, a video isencoded according to a limited encoding method based on a macroblockhaving a predetermined size.

SUMMARY

The exemplary embodiments provide a method and apparatus for encodingand decoding a video which uses an intra prediction method havingvarious directivities based on hierarchical coding units having varioussizes.

According to an exemplary embodiment, there is provided a method ofencoding an image, the method including: dividing a current picture intoat least one block having a predetermined size; determining an intraprediction mode to be applied to a current block to be encoded accordingto a size of the current block; and performing intra prediction on thecurrent block according to the determined intra prediction mode, whereinthe intra prediction mode comprises a prediction mode for performingprediction by using an extended line having an angle of tan⁻¹(dy/dx) (dxand dy are integers) about each pixel inside the current block.

According to another aspect of an exemplary embodiment, there isprovided a method of decoding an image, the method including: dividing acurrent picture into at least one block having a predetermined size;extracting information about an intra prediction mode applied to acurrent block to be decoded from a bitstream; and performing intraprediction on the current block according to the extracted intraprediction mode, wherein the intra prediction mode comprises aprediction mode for performing prediction by using an extended linehaving an angle of tan⁻¹(dy/dx) (dx and dy are integers) about eachpixel of the current block.

According to another aspect of the exemplary embodiment, there isprovided an apparatus for encoding an image, the apparatus including: anintra prediction mode determiner for determining an intra predictionmode that is to be performed according to a size of a current block tobe encoded; and an intra prediction performer for performing intraprediction on the current block to be encoded according to the intraprediction mode, wherein the intra prediction mode includes a predictionmode for performing prediction by using an extended line having an angleof tan⁻¹(dy/dx) (dx and dy are integers) about each pixel inside thecurrent block.

According to another aspect of the exemplary embodiment, there isprovided an apparatus for decoding an image, the apparatus including: anentropy decoder for extracting information about an intra predictionmode applied to a current block to be decoded from a bitstream; and anintra prediction performer for performing intra prediction on thecurrent block according to the intra prediction mode, wherein the intraprediction mode comprises a prediction mode for performing prediction byusing an extended line having an angle of tan⁻¹(dy/dx) (dx and dy areintegers) about each pixel inside the current block.

According to another aspect of the exemplary embodiment, there isprovided a computer-readable recording medium having embodied thereon aprogram for executing the method.

According to another aspect of the exemplary embodiment, there isprovided a computer-readable recording medium having embodied thereon aprogram for executing the method.

According to the exemplary embodiment, image compression efficiency maybe improved by performing intra prediction encoding in variousdirections on coding units having various sizes.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the exemplary embodiment will becomemore apparent by describing in detail exemplary embodiments thereof withreference to the attached drawings in which:

FIG. 1 is a block diagram of an apparatus for encoding a video,according to an exemplary embodiment;

FIG. 2 is a block diagram of an apparatus for decoding a video,according to an exemplary embodiment;

FIG. 3 is a diagram for explaining a concept of coding units accordingto an exemplary embodiment;

FIG. 4 is a block diagram of an image encoder based on coding unitsaccording to an exemplary embodiment;

FIG. 5 is a block diagram of an image decoder based on coding unitsaccording to an exemplary embodiment;

FIG. 6 is a diagram illustrating deeper coding units according todepths, and partitions according to an exemplary embodiment;

FIG. 7 is a diagram for explaining a relationship between a coding unitand transform units, according to an exemplary embodiment;

FIG. 8 is a diagram for explaining encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment;

FIG. 9 is a diagram of deeper coding units according to depths,according to an exemplary embodiment;

FIGS. 10 through 12 are diagrams for explaining a relationship betweencoding units, prediction units, and transform units, according to anexemplary embodiment;

FIG. 13 is a diagram for explaining a relationship between a codingunit, a prediction unit or a partition, and a transform unit, accordingto encoding mode information of Table 1;

FIG. 14 illustrates a number of intra prediction modes according to asize of a coding unit, according to an exemplary embodiment;

FIGS. 15A through 15C are diagrams for explaining an intra predictionmode applied to a coding unit having a predetermined size, according toan exemplary embodiment;

FIG. 16 is a diagram for explaining an intra prediction mode applied toa coding unit having a predetermined size, according to anotherexemplary embodiment;

FIG. 17 is a reference diagram for explaining intra prediction modeshaving various directivities, according to an exemplary embodiment;

FIG. 18 is a reference diagram for explaining a bilinear mode accordingto an exemplary embodiment;

FIG. 19 is a diagram for explaining a process of generating a predictionvalue of an intra prediction mode of a current coding unit, according toan exemplary embodiment;

FIG. 20 is a reference diagram for explaining a mapping process ofunifying intra prediction modes of coding units having different sizes,according to an exemplary embodiment;

FIG. 21 is a reference diagram for explaining a process of mapping intraprediction modes of neighboring coding units to one of representativeintra prediction modes, according to an exemplary embodiment;

FIG. 22 is a block diagram of an intra prediction apparatus according toan exemplary embodiment;

FIG. 23 is a flowchart illustrating a method of encoding an image,according to an exemplary embodiment;

FIG. 24 is a flowchart illustrating a method of decoding an image,according to an exemplary embodiment;

FIG. 25 is a diagram for explaining a relationship between a currentpixel and neighboring pixels located on an extended line having adirectivity of (dx, dy);

FIG. 26 is a diagram for explaining a change in a neighboring pixellocated on an extended line having a directivity of (dx, dy) accordingto a location of a current pixel, according to an exemplary embodiment;and

FIGS. 27 and 28 are diagrams for explaining a method of determining anintra prediction mode direction, according to exemplary embodiments.

FIG. 29 is a diagram illustrating a current coding unit and neighboringpixels to be used for intra prediction, according to an exemplaryembodiment.

DETAILED DESCRIPTION

Hereinafter, the exemplary embodiments will be described more fully withreference to the accompanying drawings, in which exemplary embodimentsare shown.

In the present specification, a ‘coding unit’ is an encoding data unitin which image data is encoded at an encoder side and an encoded dataunit in which the encoded image data is decoded at a decoder side,according to exemplary embodiments. Also, a ‘coded depth’ means a depthwhere a coding unit is encoded. Also, video comprises a still image andmoving picture. In the exemplary embodiments, “unit” may or may notrefer to a unit of size, depending on its context.

Firstly, a method and apparatus for encoding video and a method andapparatus for decoding video, according to an exemplary embodiment, willbe described with reference to FIGS. 1 to 13.

FIG. 1 is a block diagram of an apparatus 100 for encoding a video,according to an exemplary embodiment.

The apparatus 100 includes a maximum coding unit splitter 110, a codingunit determiner 120, and an output unit 130.

The maximum coding unit splitter 110 may split a current picture basedon a maximum coding unit for the current picture of an image. If thecurrent picture is larger than the maximum coding unit, image data ofthe current picture may be split into the at least one maximum codingunit. The maximum coding unit according to an exemplary embodiment maybe a data unit having a size of 32×32, 64×64, 128×128, 256×256, etc.,wherein a shape of the data unit is a square having a width and heightin squares of 2. The image data may be output to the coding unitdeterminer 120 according to the at least one maximum coding unit.

A coding unit according to an exemplary embodiment may be characterizedby a maximum size and a depth. The depth denotes a number of times thecoding unit is spatially split from the maximum coding unit, and as thedepth deepens or increases, deeper encoding units according to depthsmay be split from the maximum coding unit to a minimum coding unit. Adepth of the maximum coding unit is an uppermost depth and a depth ofthe minimum coding unit is a lowermost depth. Since a size of a codingunit corresponding to each depth decreases as the depth of the maximumcoding unit deepens, a coding unit corresponding to an upper depth mayinclude a plurality of coding units corresponding to lower depths.

As described above, the image data of the current picture is split intothe maximum coding units according to a maximum size of the coding unit,and each of the maximum coding units may include deeper coding unitsthat are split according to depths. Since the maximum coding unitaccording to an exemplary embodiment is split according to depths, theimage data of a spatial domain included in the maximum coding unit maybe hierarchically classified according to depths.

A maximum depth and a maximum size of a coding unit, which limit thetotal number of times a height and a width of the maximum coding unitare hierarchically split may be predetermined.

The coding unit determiner 120 encodes at least one split regionobtained by splitting a region of the maximum coding unit according todepths, and determines a depth to output a finally encoded image dataaccording to the at least one split region. In other words, the codingunit determiner 120 determines a coded depth by encoding the image datain the deeper coding units according to depths, according to the maximumcoding unit of the current picture, and selects a depth having the leastencoding error. Thus, the encoded image data of the coding unitcorresponding to the determined coded depth are finally output. Also,the coding units corresponding to the coded depth may be regarded asencoded coding units.

The determined coded depth and the encoded image data according to thedetermined coded depth are output to the output unit 130.

The image data in the maximum coding unit is encoded based on the deepercoding units corresponding to at least one depth equal to or below themaximum depth, and results of encoding the image data are compared basedon each of the deeper coding units. A depth having the least encodingerror may be selected after comparing encoding errors of the deepercoding units. At least one coded depth may be selected for each maximumcoding unit.

The size of the maximum coding unit is split as a coding unit ishierarchically split according to depths, and as the number of codingunits increases. Also, even if coding units corresponding to same depthin one maximum coding unit, each of the coding units corresponding tothe same depth may be split to a lower depth by measuring an encodingerror of the image data of the each coding unit, separately.Accordingly, even when image data is included in one maximum codingunit, the image data is split to regions according to the depths, theencoding errors may differ according to regions in the one maximumcoding unite, and thus the coded depths may differ according to regionsin the image data. Thus, one or more coded depths may be determined inone maximum coding unit, and the image data of the maximum coding unitmay be divided according to coding units of at least one coded depth.

Accordingly, the coding unit determiner 120 may determine coding unitshaving a tree structure included in the maximum coding unit. The ‘codingunits having a tree structure’ according to an exemplary embodimentinclude coding units corresponding to a depth determined to be the codeddepth, from among all deeper coding units included in the maximum codingunit. A coding unit of a coded depth may be hierarchically determinedaccording to depths in the same region of the maximum coding unit, andmay be independently determined in different regions. Similarly, a codeddepth in a current region may be independently determined from a codeddepth in another region.

A maximum depth according to an exemplary embodiment is an index relatedto the number of splitting times from a maximum coding unit to a minimumcoding unit. A first maximum depth according to an exemplary embodimentmay denote the total number of splitting times from the maximum codingunit to the minimum coding unit. A second maximum depth according to anexemplary embodiment may denote the total number of depth levels fromthe maximum coding unit to the minimum coding unit. For example, when adepth of the maximum coding unit is 0, a depth of a coding unit, inwhich the maximum coding unit is split once, may be set to 1, and adepth of a coding unit, in which the maximum coding unit is split twice,may be set to 2. Here, if the minimum coding unit is a coding unit inwhich the maximum coding unit is split four times, 5 depth levels ofdepths 0, 1, 2, 3 and 4 exist, and thus the first maximum depth may beset to 4, and the second maximum depth may be set to 5.

Prediction encoding and transformation may be performed according to themaximum coding unit. The prediction encoding and the transformation arealso performed based on the deeper coding units according to a depthequal to or depths less than the maximum depth, according to the maximumcoding unit. Transformation may be performed according to method oforthogonal transformation or integer transformation.

Since the number of deeper coding units increases whenever the maximumcoding unit is split according to depths, encoding including theprediction encoding and the transformation is performed on all of thedeeper coding units generated as the depth deepens. For convenience ofdescription, the prediction encoding and the transformation will now bedescribed based on a coding unit of a current depth, in a maximum codingunit.

The apparatus 100 may variously select a size or shape of a data unitfor encoding the image data. In order to encode the image data,operations, such as prediction encoding, transformation, and entropyencoding, are performed, and at this time, the same data unit may beused for all operations or different data units may be used for eachoperation.

For example, the apparatus 100 may select not only a coding unit forencoding the image data, but also a data unit different from the codingunit so as to perform the prediction encoding on the image data in thecoding unit.

In order to perform prediction encoding in the maximum coding unit, theprediction encoding may be performed based on a coding unitcorresponding to a coded depth, i.e., based on a coding unit that is nolonger split to coding units corresponding to a lower depth.Hereinafter, the coding unit that is no longer split and becomes a basisunit for prediction encoding will now be referred to as a ‘predictionunit’. A partition obtained by splitting the prediction unit may includea data unit obtained by splitting at least one of a height and a widthof the prediction unit.

For example, when a coding unit of 2N×2N (where N is a positive integer)is no longer split and becomes a prediction unit of 2N×2N, and a size ofa partition may be 2N×2N, 2N×N, N×2N, or N×N. Examples of a partitiontype include symmetrical partitions that are obtained by symmetricallysplitting a height or width of the prediction unit, partitions obtainedby asymmetrically splitting the height or width of the prediction unit,such as 1:n or n:1, partitions that are obtained by geometricallysplitting the prediction unit, and partitions having arbitrary shapes.

A prediction mode of the prediction unit may be at least one of an intramode, a inter mode, and a skip mode. For example, the intra mode or theinter mode may be performed on the partition of 2N×2N, 2N×N, N×2N, orN×N. Also, the skip mode may be performed only on the partition of2N×2N. The encoding is independently performed on one prediction unit ina coding unit, thereby selecting a prediction mode having a leastencoding error.

The apparatus 100 may also perform the transformation on the image datain a coding unit based not only on the coding unit for encoding theimage data, but also based on a data unit that is different from thecoding unit.

In order to perform the transformation in the coding unit, thetransformation may be performed based on a data unit having a sizesmaller than or equal to the coding unit. For example, the data unit forthe transformation may include a data unit for an intra mode and a dataunit for an inter mode.

A data unit used as a base of the transformation will now be referred toas a ‘transform unit’. A transformation depth indicating the number ofsplitting times to reach the transform unit by splitting the height andwidth of the coding unit may also be set in the transform unit. Forexample, in a current coding unit of 2N×2N, a transformation depth maybe 0 when the size of a transform unit is also 2N×2N, may be 1 when eachof the height and width of the current coding unit is split into twoequal parts, totally split into 4^1 transform units, and the size of thetransform unit is thus N×N, and may be 2 when each of the height andwidth of the current coding unit is split into four equal parts, totallysplit into 4^2 transform units and the size of the transform unit isthus N/2×N/2. For example, the transform unit may be set according to ahierarchical tree structure, in which a transform unit of an uppertransformation depth is split into four transform units of a lowertransformation depth according to the hierarchical characteristics of atransformation depth.

Similarly to the coding unit, the transform unit in the coding unit maybe recursively split into smaller sized regions, so that the transformunit may be determined independently in units of regions. Thus, residualdata in the coding unit may be divided according to the transformationhaving the tree structure according to transformation depths.

Encoding information according to coding units corresponding to a codeddepth requires not only information about the coded depth, but alsoabout information related to prediction encoding and transformation.Accordingly, the coding unit determiner 120 not only determines a codeddepth having a least encoding error, but also determines a partitiontype in a prediction unit, a prediction mode according to predictionunits, and a size of a transform unit for transformation.

Coding units according to a tree structure in a maximum coding unit anda method of determining a partition, according to exemplary embodiments,will be described in detail later with reference to FIGS. 3 through 12.

The coding unit determiner 120 may measure an encoding error of deepercoding units according to depths by using Rate-Distortion Optimizationbased on Lagrangian multipliers.

The output unit 130 outputs the image data of the maximum coding unit,which is encoded based on the at least one coded depth determined by thecoding unit determiner 120, and information about the encoding modeaccording to the coded depth, in bitstreams.

The encoded image data may be obtained by encoding residual data of animage.

The information about the encoding mode according to coded depth mayinclude information about the coded depth, about the partition type inthe prediction unit, the prediction mode, and the size of the transformunit.

The information about the coded depth may be defined by using splitinformation according to depths, which indicates whether encoding isperformed on coding units of a lower depth instead of a current depth.If the current depth of the current coding unit is the coded depth,image data in the current coding unit is encoded and output, and thusthe split information may be defined not to split the current codingunit to a lower depth. Alternatively, if the current depth of thecurrent coding unit is not the coded depth, the encoding is performed onthe coding unit of the lower depth, and thus the split information maybe defined to split the current coding unit to obtain the coding unitsof the lower depth.

If the current depth is not the coded depth, encoding is performed onthe coding unit that is split into the coding unit of the lower depth.Since at least one coding unit of the lower depth exists in one codingunit of the current depth, the encoding is repeatedly performed on eachcoding unit of the lower depth, and thus the encoding may be recursivelyperformed for the coding units having the same depth.

Since the coding units having a tree structure are determined for onemaximum coding unit, and information about at least one encoding mode isdetermined for a coding unit of a coded depth, information about atleast one encoding mode may be determined for one maximum coding unit.Also, a coded depth of the image data of the maximum coding unit may bedifferent according to locations since the image data is hierarchicallysplit according to depths, and thus information about the coded depthand the encoding mode may be set for the image data.

Accordingly, the output unit 130 may assign encoding information about acorresponding coded depth and an encoding mode to at least one of thecoding unit, the prediction unit, and a minimum unit included in themaximum coding unit.

The minimum unit according to an exemplary embodiment is a rectangulardata unit obtained by splitting the minimum coding unit constituting thelowermost depth by 4. Alternatively, the minimum unit may be a maximumrectangular data unit that may be included in all of the coding units,prediction units, partition units, and transform units included in themaximum coding unit.

For example, the encoding information output through the output unit 130may be classified into encoding information according to coding units,and encoding information according to prediction units. The encodinginformation according to the coding units may include the informationabout the prediction mode and about the size of the partitions. Theencoding information according to the prediction units may includeinformation about an estimated direction of an inter mode, about areference image index of the inter mode, about a motion vector, about achroma component of an intra mode, and about an interpolation method ofthe intra mode. Also, information about a maximum size of the codingunit defined according to pictures, slices, or GOPs, and informationabout a maximum depth may be inserted into SPS (Sequence Parameter Set)or a header of a bitstream.

In the apparatus 100, the deeper coding unit may be a coding unitobtained by dividing a height or width of a coding unit of an upperdepth by two. In other words, when the size of the coding unit of thecurrent depth is 2N×2N, the size of the coding unit of the lower depthis N×N. Also, the coding unit of the current depth having the size of2N×2N may include maximum 4 of the coding unit of the lower depth.

Accordingly, the apparatus 100 may form the coding units having the treestructure by determining coding units having an optimum shape and anoptimum size for each maximum coding unit, based on the size of themaximum coding unit and the maximum depth determined consideringcharacteristics of the current picture. Also, since encoding may beperformed on each maximum coding unit by using any one of variousprediction modes and transformations, an optimum encoding mode may bedetermined considering characteristics of the coding unit of variousimage sizes.

Thus, if an image having high resolution or large data amount is encodedin a conventional macroblock, a number of macroblocks per pictureexcessively increases. Accordingly, a number of pieces of compressedinformation generated for each macroblock increases, and thus it isdifficult to transmit the compressed information and data compressionefficiency decreases. However, by using the apparatus 100, imagecompression efficiency may be increased since a coding unit is adjustedwhile considering characteristics of an image while increasing a maximumsize of a coding unit while considering a size of the image.

FIG. 2 is a block diagram of an apparatus 200 for decoding a video,according to an exemplary embodiment.

The apparatus 200 includes a receiver 210, an image data and encodinginformation extractor 220, and an image data decoder 230. Definitions ofvarious terms, such as a coding unit, a depth, a prediction unit, atransform unit, and information about various encoding modes, forvarious operations of the apparatus 200 are identical to those describedwith reference to FIG. 1 and the apparatus 100.

The receiver 210 receives and parses a bitstream of an encoded video.The image data and encoding information extractor 220 extracts encodedimage data for each coding unit from the parsed bitstream, wherein thecoding units have a tree structure according to each maximum codingunit, and outputs the extracted image data to the image data decoder230. The image data and encoding information extractor 220 may extractinformation about a maximum size of a coding unit of a current picture,from a header about the current picture or SPS.

Also, the image data and encoding information extractor 220 extractsinformation about a coded depth and an encoding mode for the codingunits having a tree structure according to each maximum coding unit,from the parsed bitstream. The extracted information about the codeddepth and the encoding mode is output to the image data decoder 230. Inother words, the image data in a bit stream is split into the maximumcoding unit so that the image data decoder 230 decodes the image datafor each maximum coding unit.

The information about the coded depth and the encoding mode according tothe maximum coding unit may be set for information about at least onecoding unit corresponding to the coded depth, and information about anencoding mode may include information about a partition type of acorresponding coding unit corresponding to the coded depth, about aprediction mode, and a size of a transform unit. Also, splittinginformation according to depths may be extracted as the informationabout the coded depth.

The information about the coded depth and the encoding mode according toeach maximum coding unit extracted by the image data and encodinginformation extractor 220 is information about a coded depth and anencoding mode determined to generate a minimum encoding error when anencoder, such as the apparatus 100, repeatedly performs encoding foreach deeper coding unit according to depths according to each maximumcoding unit. Accordingly, the apparatus 200 may restore an image bydecoding the image data according to a coded depth and an encoding modethat generates the minimum encoding error.

Since encoding information about the coded depth and the encoding modemay be assigned to a predetermined data unit from among a correspondingcoding unit, a prediction unit, and a minimum unit, the image data andencoding information extractor 220 may extract the information about thecoded depth and the encoding mode according to the predetermined dataunits. The predetermined data units to which the same information aboutthe coded depth and the encoding mode is assigned may be inferred to bethe data units included in the same maximum coding unit.

The image data decoder 230 restores the current picture by decoding theimage data in each maximum coding unit based on the information aboutthe coded depth and the encoding mode according to the maximum codingunits. In other words, the image data decoder 230 may decode the encodedimage data based on the extracted information about the partition type,the prediction mode, and the transform unit for each coding unit fromamong the coding units having the tree structure included in eachmaximum coding unit. A decoding process may include a predictionincluding intra prediction and motion compensation, and a inversetransformation. Inverse transformation may be performed according tomethod of inverse orthogonal transformation or inverse integertransformation.

The image data decoder 230 may perform intra prediction or motioncompensation according to a partition and a prediction mode of eachcoding unit, based on the information about the partition type and theprediction mode of the prediction unit of the coding unit according tocoded depths.

Also, the image data decoder 230 may perform inverse transformationaccording to each transform unit in the coding unit, based on theinformation about the size of the transform unit of the coding unitaccording to coded depths, so as to perform the inverse transformationaccording to maximum coding units.

The image data decoder 230 may determine at least one coded depth of acurrent maximum coding unit by using split information according todepths. If the split information indicates that image data is no longersplit in the current depth, the current depth is a coded depth.Accordingly, the image data decoder 230 may decode encoded data of atleast one coding unit corresponding to the each coded depth in thecurrent maximum coding unit by using the information about the partitiontype of the prediction unit, the prediction mode, and the size of thetransform unit for each coding unit corresponding to the coded depth,and output the image data of the current maximum coding unit.

In other words, data units containing the encoding information includingthe same split information may be gathered by observing the encodinginformation set assigned for the predetermined data unit from among thecoding unit, the prediction unit, and the minimum unit, and the gathereddata units may be considered to be one data unit to be decoded by theimage data decoder 230 in the same encoding mode.

The apparatus 200 may obtain information about at least one coding unitthat generates the minimum encoding error when encoding is recursivelyperformed for each maximum coding unit, and may use the information todecode the current picture. In other words, the coding units having thetree structure determined to be the optimum coding units in each maximumcoding unit may be decoded. Also, the maximum size of coding unit isdetermined considering resolution and a amount of image data.

Accordingly, even if image data has high resolution and a large amountof data, the image data may be efficiently decoded and restored by usinga size of a coding unit and an encoding mode, which are adaptivelydetermined according to characteristics of the image data, by usinginformation about an optimum encoding mode received from an encoder.

A method of determining coding units having a tree structure, aprediction unit, and a transform unit, according to an exemplaryembodiment, will now be described with reference to FIGS. 3 through 13.

FIG. 3 is a diagram for explaining a concept of coding units accordingto an exemplary embodiment.

A size of a coding unit may be expressed in width×height, and may be64×64, 32×32, 16×16, and 8×8. A coding unit of 64×64 may be split intopartitions of 64×64, 64×32, 32×64, or 32×32, and a coding unit of 32×32may be split into partitions of 32×32, 32×16, 16×32, or 16×16, a codingunit of 16×16 may be split into partitions of 16×16, 16×8, 8×16, or 8×8,and a coding unit of 8×8 may be split into partitions of 8×8, 8×4, 4×8,or 4×4.

In video data 310, a resolution is 1920×1080, a maximum size of a codingunit is 64, and a maximum depth is 2. In video data 320, a resolution is1920×1080, a maximum size of a coding unit is 64, and a maximum depth is3. In video data 330, a resolution is 352×288, a maximum size of acoding unit is 16, and a maximum depth is 1. The maximum depth shown inFIG. 3 denotes a total number of splits from a maximum coding unit to aminimum decoding unit.

If a resolution is high or a data amount is large, a maximum size of acoding unit may be large so as to not only increase encoding efficiencybut also to accurately reflect characteristics of an image. Accordingly,the maximum size of the coding unit of the video data 310 and 320 havingthe higher resolution than the video data 330 may be 64.

Since the maximum depth of the video data 310 is 2, coding units 315 ofthe vide data 310 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32 and 16 sincedepths are deepened to two layers by splitting the maximum coding unittwice. Meanwhile, since the maximum depth of the video data 330 is 1,coding units 335 of the video data 330 may include a maximum coding unithaving a long axis size of 16, and coding units having a long axis sizeof 8 since depths are deepened to one layer by splitting the maximumcoding unit once.

Since the maximum depth of the video data 320 is 3, coding units 325 ofthe video data 320 may include a maximum coding unit having a long axissize of 64, and coding units having long axis sizes of 32, 16, and 8since the depths are deepened to 3 layers by splitting the maximumcoding unit three times. As a depth deepens, detailed information may beprecisely expressed.

FIG. 4 is a block diagram of an image encoder 400 based on coding units,according to an exemplary embodiment.

The image encoder 400 performs operations of the coding unit determiner120 of the apparatus 100 to encode image data. In other words, an intrapredictor 410 performs intra prediction on coding units in an intramode, from among a current frame 405, and a motion estimator 420 and amotion compensator 425 performs inter estimation and motion compensationon coding units in an inter mode from among the current frame 405 byusing the current frame 405, and a reference frame 495.

Data output from the intra predictor 410, the motion estimator 420, andthe motion compensator 425 is output as a quantized transformationcoefficient through a transformer 430 and a quantizer 440. The quantizedtransformation coefficient is restored as data in a spatial domainthrough an inverse quantizer 460 and an inverse transformer 470, and therestored data in the spatial domain is output as the reference frame 495after being post-processed through a deblocking unit 480 and a loopfiltering unit 490. The quantized transformation coefficient may beoutput as a bitstream 455 through an entropy encoder 450.

In order for the image encoder 400 to be applied in the apparatus 100,all elements of the image encoder 400, i.e., the intra predictor 410,the motion estimator 420, the motion compensator 425, the transformer430, the quantizer 440, the entropy encoder 450, the inverse quantizer460, the inverse transformer 470, the deblocking unit 480, and the loopfiltering unit 490 perform operations based on each coding unit fromamong coding units having a tree structure while considering the maximumdepth of each maximum coding unit.

Specifically, the intra predictor 410, the motion estimator 420, and themotion compensator 425 determines partitions and a prediction mode ofeach coding unit from among the coding units having a tree structurewhile considering the maximum size and the maximum depth of a currentmaximum coding unit, and the transformer 430 determines the size of thetransform unit in each coding unit from among the coding units having atree structure.

FIG. 5 is a block diagram of an image decoder 500 based on coding units,according to an exemplary embodiment.

A parser 510 parses encoded image data to be decoded and informationabout encoding required for decoding from a bitstream 505. The encodedimage data is output as inverse quantized data through an entropydecoder 520 and an inverse quantizer 530, and the inverse quantized datais restored to image data in a spatial domain through an inversetransformer 540.

An intra predictor 550 performs intra prediction on coding units in anintra mode with respect to the image data in the spatial domain, and amotion compensator 560 performs motion compensation on coding units inan inter mode by using a reference frame 585.

The image data in the spatial domain, which passed through the intrapredictor 550 and the motion compensator 560, may be output as arestored frame 595 after being post-processed through a deblocking unit570 and a loop filtering unit 580. Also, the image data that ispost-processed through the deblocking unit 570 and the loop filteringunit 580 may be output as the reference frame 585.

In order to decode the image data in the image data decoder 230 of theapparatus 200, the image decoder 500 may perform operations that areperformed after the parser 510.

In order for the image decoder 500 to be applied in the apparatus 200,all elements of the image decoder 500, i.e., the parser 510, the entropydecoder 520, the inverse quantizer 530, the inverse transformer 540, theintra predictor 550, the motion compensator 560, the deblocking unit570, and the loop filtering unit 580 perform operations based on codingunits having a tree structure for each maximum coding unit.

Specifically, the intra prediction 550 and the motion compensator 560perform operations based on partitions and a prediction mode for each ofthe coding units having a tree structure, and the inverse transformer540 perform operations based on a size of a transform unit for eachcoding unit.

FIG. 6 is a diagram illustrating deeper coding units according todepths, and partitions, according to an exemplary embodiment.

The apparatus 100 and the apparatus 200 use hierarchical coding units soas to consider characteristics of an image. A maximum height, a maximumwidth, and a maximum depth of coding units may be adaptively determinedaccording to the characteristics of the image, or may be differently setby a user. Sizes of deeper coding units according to depths may bedetermined according to the predetermined maximum size of the codingunit.

In a hierarchical structure 600 of coding units, according to anexemplary embodiment, the maximum height and the maximum width of thecoding units are each 64, and the maximum depth is 4. Since a depthdeepens along a vertical axis of the hierarchical structure 600, aheight and a width of the deeper coding unit are each split. Also, aprediction unit and partitions, which are bases for prediction encodingof each deeper coding unit, are shown along a horizontal axis of thehierarchical structure 600.

In other words, a coding unit 610 is a maximum coding unit in thehierarchical structure 600, wherein a depth is 0 and a size, i.e., aheight by width, is 64×64. The depth deepens along the vertical axis,and a coding unit 620 having a size of 32×32 and a depth of 1, a codingunit 630 having a size of 16×16 and a depth of 2, a coding unit 640having a size of 8×8 and a depth of 3, and a coding unit 650 having asize of 4×4 and a depth of 4 exist. The coding unit 650 having the sizeof 4×4 and the depth of 4 is a minimum coding unit.

The prediction unit and the partitions of a coding unit are arrangedalong the horizontal axis according to each depth. In other words, ifthe coding unit 610 having the size of 64×64 and the depth of 0 is aprediction unit, the prediction unit may be split into partitionsinclude in the encoding unit 610, i.e. a partition 610 having a size of64×64, partitions 612 having the size of 64×32, partitions 614 havingthe size of 32×64, or partitions 616 having the size of 32×32.

Similarly, a prediction unit of the coding unit 620 having the size of32×32 and the depth of 1 may be split into partitions included in thecoding unit 620, i.e. a partition 620 having a size of 32×32, partitions622 having a size of 32×16, partitions 624 having a size of 16×32, andpartitions 626 having a size of 16×16.

Similarly, a prediction unit of the coding unit 630 having the size of16×16 and the depth of 2 may be split into partitions included in thecoding unit 630, i.e. a partition having a size of 16×16 included in thecoding unit 630, partitions 632 having a size of 16×8, partitions 634having a size of 8×16, and partitions 636 having a size of 8×8.

Similarly, a prediction unit of the coding unit 640 having the size of8×8 and the depth of 3 may be split into partitions included in thecoding unit 640, i.e. a partition having a size of 8×8 included in thecoding unit 640, partitions 642 having a size of 8×4, partitions 644having a size of 4×8, and partitions 646 having a size of 4×4.

The coding unit 650 having the size of 4×4 and the depth of 4 is theminimum coding unit and a coding unit of the lowermost depth. Aprediction unit of the coding unit 650 is only assigned to a partitionhaving a size of 4×4.

In order to determine the at least one coded depth of the coding unitsconstituting the maximum coding unit 610, the coding unit determiner 120of the apparatus 100 performs encoding for coding units corresponding toeach depth included in the maximum coding unit 610.

A number of deeper coding units according to depths including data inthe same range and the same size increases as the depth deepens. Forexample, four coding units corresponding to a depth of 2 are required tocover data that is included in one coding unit corresponding to a depthof 1. Accordingly, in order to compare encoding results of the same dataaccording to depths, the coding unit corresponding to the depth of 1 andfour coding units corresponding to the depth of 2 are each encoded.

In order to perform encoding for a current depth from among the depth, aleast encoding error may be selected for the current depth by performingencoding for each prediction unit in the coding units corresponding tothe current depth, along the horizontal axis of the hierarchicalstructure 600. Alternatively, the minimum encoding error may be searchedfor by comparing the least encoding errors according to depths, byperforming encoding for each depth as the depth deepens along thevertical axis of the hierarchical structure 600. A depth and a partitionhaving the minimum encoding error in the coding unit 610 may be selectedas the coded depth and a partition type of the coding unit 610.

FIG. 7 is a diagram for explaining a relationship between a coding unit710 and transform units 720, according to an exemplary embodiment.

The apparatus 100 or 200 encodes or decodes an image according to codingunits having sizes smaller than or equal to a maximum coding unit foreach maximum coding unit. Sizes of transform units for transformationduring encoding may be selected based on data units that are not largerthan corresponding coding unit.

For example, in the apparatus 100 or 200, if a size of the coding unit710 is 64×64, transformation may be performed by using the transformunits 720 having a size of 32×32.

Also, data of the coding unit 710 having the size of 64×64 may beencoded by performing the transformation on each of the transform unitshaving the size of 32×32, 16×16, 8×8, and 4×4, which are smaller than64×64, and then a transform unit having the least coding error may beselected.

FIG. 8 is a diagram for explaining encoding information of coding unitscorresponding to a coded depth, according to an exemplary embodiment.

The output unit 130 of the apparatus 100 may encode and transmitinformation 800 about a partition type, information 810 about aprediction mode, and information 820 about a size of a transform unitfor each coding unit corresponding to a coded depth, as informationabout an encoding mode.

The information 800 indicates information about a shape of a partitionobtained by splitting a prediction unit of a current coding unit,wherein the partition is a data unit for prediction encoding the currentcoding unit. For example, a current coding unit CU_0 having a size of2N×2N may be split into any one of a partition 802 having a size of2N×2N, a partition 804 having a size of 2N×N, a partition 806 having asize of N×2N, and a partition 808 having a size of N×N. Here, theinformation 800 about a partition type is set to indicate one of thepartition 804 having a size of 2N×N, the partition 806 having a size ofN×2N, and the partition 808 having a size of N×N

The information 810 indicates a prediction mode of each partition. Forexample, the information 810 may indicate a mode of prediction encodingperformed on a partition indicated by the information 800, i.e., anintra mode 812, an inter mode 814, or a skip mode 816.

The information 820 indicates a transform unit to be based on whentransformation is performed on a current coding unit. For example, thetransform unit may be a first intra transform unit 822, a second intratransform unit 824, a first inter transform unit 826, or a second intratransform unit 828.

The image data and encoding information extractor 220 of the apparatus200 may extract and use the information 800, 810, and 820 for decoding.

FIG. 9 is a diagram of deeper coding units according to depths,according to an exemplary embodiment.

Split information may be used to indicate a change of a depth. The spiltinformation indicates whether a coding unit of a current depth is splitinto coding units of a lower depth.

A prediction unit 910 for prediction encoding a coding unit 900 having adepth of 0 and a size of 2N_0×2N_0 may include partitions of a partitiontype 912 having a size of 2N_0×2N_0, a partition type 914 having a sizeof 2N_0×N_0, a partition type 916 having a size of N_0×2N_0, and apartition type 918 having a size of N_0×N_0. FIG. 9 only illustrates thepartition types 912 through 918 which are obtained by symmetricallysplitting the prediction unit 910, but a partition type is not limitedthereto, and the partitions of the prediction unit 910 may includeasymmetrical partitions, partitions having a predetermined shape, andpartitions having a geometrical shape.

Prediction encoding is repeatedly performed on one partition having asize of 2N_0×2N_0, two partitions having a size of 2N_0×N_0, twopartitions having a size of N_0×2N_0, and four partitions having a sizeof N_0×N_0, according to each partition type. The prediction encoding inan intra mode and an inter mode may be performed on the partitionshaving the sizes of 2N_0×2N_0, N_0×2N_0, 2N_0×N_0, and N_0×N_0. Theprediction encoding in a skip mode is performed only on the partitionhaving the size of 2N_0×2N_0.

Errors of encoding including the prediction encoding in the partitiontypes 912 through 918 are compared, and the least encoding error isdetermined among the partition types. If an encoding error is smallestin one of the partition types 912 through 916, the prediction unit 910may not be split into a lower depth.

If the encoding error is the smallest in the partition type 918, a depthis changed from 0 to 1 to split the partition type 918 in operation 920,and encoding is repeatedly performed on coding units 930 having a depthof 2 and a size of N_0×N_0 to search for a minimum encoding error.

A prediction unit 940 for prediction encoding the coding unit 930 havinga depth of 1 and a size of 2N_1×2N_1 (=N_0×N_0) may include partitionsof a partition type 942 having a size of 2N_1×2N_1, a partition type 944having a size of 2N_1×N_1, a partition type 946 having a size ofN_1×2N_1, and a partition type 948 having a size of N_1×N_1.

If an encoding error is the smallest in the partition type 948, a depthis changed from 1 to 2 to split the partition type 948 in operation 950,and encoding is repeatedly performed on coding units 960, which have adepth of 2 and a size of N_2×N_2 to search for a minimum encoding error.

When a maximum depth is d, split operation according to each depth maybe performed up to when a depth becomes d-1, and split information maybe encoded as up to when a depth is one of 0 to d-2. In other words,when encoding is performed up to when the depth is d-1 after a codingunit corresponding to a depth of d-2 is split in operation 970, aprediction unit 990 for prediction encoding a coding unit 980 having adepth of d-1 and a size of 2N_(d-1)×2N_(d-1) may include partitions of apartition type 992 having a size of 2N_(d-1)×2N_(d-1), a partition type994 having a size of 2N_(d-1)×N_(d-1), a partition type 996 having asize of N (d-1)×2N (d-1), and a partition type 998 having a size ofN_(d-1)×N_(d-1).

Prediction encoding may be repeatedly performed on one partition havinga size of 2N_(d-1)×2N (d-1), two partitions having a size of 2N (d-1)×N(d-1), two partitions having a size of N (d-1)×2N_(d-1), four partitionshaving a size of N_(d-1)×N_(d-1) from among the partition types 992through 998 to search for a partition type having a minimum encodingerror.

Even when the partition type 998 has the minimum encoding error, since amaximum depth is d, a coding unit CU_(d-1) having a depth of d-1 is nolonger split to a lower depth, and a coded depth for the coding unitsconstituting a current maximum coding unit 900 is determined to be d-1and a partition type of the coding unit 900 may be determined to beN_(d-1)×N_(d-1). Also, since the maximum depth is d and a minimum codingunit 980 having a lowermost depth of d-1 is no longer split to a lowerdepth, split information for a coding unit 980 is not set.

A data unit 999 may be a ‘minimum unit’ for the current maximum codingunit. A minimum unit according to an exemplary embodiment may be arectangular data unit obtained by splitting a minimum coding unit 980 by4. By performing the encoding repeatedly, the apparatus 100 may select adepth having the least encoding error by comparing encoding errorsaccording to depths of the coding unit 900 to determine a coded depth,and set a corresponding partition type and a prediction mode as anencoding mode of the coded depth.

As such, the minimum encoding errors according to depths are compared inall of the depths of 1 through d, and a depth having the least encodingerror may be determined as a coded depth. The coded depth, the partitiontype of the prediction unit, and the prediction mode may be encoded andtransmitted as information about an encoding mode. Also, since a codingunit is split from a depth of 0 to a coded depth, only split informationof the coded depth is set to 0, and split information of depthsexcluding the coded depth is set to 1.

The image data and encoding information extractor 220 of the apparatus200 may extract and use the information about the coded depth and theprediction unit of the coding unit 900 to decode the partition 912. Theapparatus 200 may determine a depth, in which split information is 0, asa coded depth by using split information according to depths, and useinformation about an encoding mode of the corresponding depth fordecoding.

FIGS. 10 through 12 are diagrams for explaining a relationship betweencoding units 1010, prediction units 1060, and transform units 1070,according to an exemplary embodiment.

The coding units 1010 are coding units having a tree structure,corresponding to coded depths determined by the apparatus 100, in amaximum coding unit. The prediction units 1060 are partitions ofprediction units of each of the coding units 1010, and the transformunits 1070 are transform units of each of the coding units 1010.

When a depth of a maximum coding unit is 0 in the coding units 1010,depths of coding units 1012 and 1054 are 1, depths of coding units 1014,1016, 1018, 1028, 1050, and 1052 are 2, depths of coding units 1020,1022, 1024, 1026, 1030, 1032, and 1048 are 3, and depths of coding units1040, 1042, 1044, and 1046 are 4.

In the prediction units 1060, some encoding units 1014, 1016, 1022,1032, 1048, 1050, 1052, and 1054 are split into partitions forprediction encoding. In other words, partition types in the coding units1014, 1022, 1050, and 1054 have a size of 2N×N, partition types in thecoding units 1016, 1048, and 1052 have a size of N×2N, and a partitiontype of the coding unit 1032 has a size of N×N. Prediction units andpartitions of the coding units 1010 are smaller than or equal to eachcoding unit.

Transformation or inverse transformation is performed on image data ofthe coding unit 1052 in the transform units 1070 in a data unit that issmaller than the coding unit 1052. Also, the coding units 1014, 1016,1022, 1032, 1048, 1050, and 1052 in the transform units 1070 aredifferent from those in the prediction units 1060 in terms of sizes andshapes. In other words, the apparatuses 100 and 200 may perform intraprediction, motion estimation, motion compensation, transformation, andinverse transformation individually on a data unit in the same codingunit.

Accordingly, encoding is recursively performed on each of coding unitshaving a hierarchical structure in each region of a maximum coding unitto determine an optimum coding unit, and thus coding units having arecursive tree structure may be obtained. Encoding information mayinclude split information about a coding unit, information about apartition type, information about a prediction mode, and informationabout a size of a transform unit. Table 1 shows the encoding informationthat may be set by the apparatuses 100 and 200.

TABLE 1 Split Information 0 (Encoding on Coding unit having Size of 2N ×2N and Current Depth of d) Partition Type Size of Transform UnitSymmetrical Asymmetrical Split Split Prediction Partition PartitionInformation 0 Information 1 Split Mode Type Type of Transform Unit ofTransform Unit Information 1 Intra 2N × 2N 2N × nU 2N × 2N N × NRepeatedly Inter 2N × N  2N × nD (Symmetrical Type) Encode Coding Skip N × 2N nL × 2N N/2 × N/2 Units having (Only 2N × 2N) N × N nR × 2N(Asymmetrical Type) Lower Depth of d + 1

The output unit 130 of the apparatus 100 may output the encodinginformation about the coding units having a tree structure, and theimage data and encoding information extractor 220 of the apparatus 200may extract the encoding information about the coding units having atree structure from a received bitstream.

Split information indicates whether a current coding unit is split intocoding units of a lower depth. If split information of a current depth dis 0, a depth, in which a current coding unit is no longer split into alower depth, is a coded depth, and thus information about a partitiontype, prediction mode, and a size of a transform unit may be defined forthe coded depth. If the current coding unit is further split accordingto the split information, encoding is independently performed on foursplit coding units of a lower depth.

A prediction mode may be one of an intra mode, an inter mode, and a skipmode. The intra mode and the inter mode may be defined in all partitiontypes, and the skip mode is defined only in a partition type having asize of 2N×2N.

The information about the partition type may indicate symmetricalpartition types having sizes of 2N×2N, 2N×N, N×2N, and N×N, which areobtained by symmetrically splitting a height or a width of a predictionunit, and asymmetrical partition types having sizes of 2N×nU, 2N×nD,nL×2N, and nR×2N, which are obtained by asymmetrically splitting theheight or width of the prediction unit. The asymmetrical partition typeshaving the sizes of 2N×nU and 2N×nD may be respectively obtained bysplitting the height of the prediction unit in 1:3 and 3:1, and theasymmetrical partition types having the sizes of nL×2N and nR×2N may berespectively obtained by splitting the width of the prediction unit in1:3 and 3:1

The size of the transform unit may be set to be two types in the intramode and two types in the inter mode. In other words, if splitinformation of the transform unit is 0, the size of the transform unitmay be 2N×2N, which is the size of the current coding unit. If splitinformation of the transform unit is 1, the transform units may beobtained by splitting the current coding unit. Also, if a partition typeof the current coding unit having the size of 2N×2N is a symmetricalpartition type, a size of a transform unit may be N×N, and if thepartition type of the current coding unit is an asymmetrical partitiontype, the size of the transform unit may be N/2×N/2.

The encoding information about coding units having a tree structure mayinclude at least one of a coding unit corresponding to a coded depth, aprediction unit, and a minimum unit. The coding unit corresponding tothe coded depth may include at least one of a prediction unit and aminimum unit containing the same encoding information.

Accordingly, it is determined whether adjacent data units are includedin the same coding unit corresponding to the coded depth by comparingencoding information of the adjacent data units. Also, a correspondingcoding unit corresponding to a coded depth is determined by usingencoding information of a data unit, and thus a distribution of codeddepths in a maximum coding unit may be determined.

Accordingly, if a current coding unit is predicted based on encodinginformation of adjacent data units, encoding information of data unitsin deeper encoding units adjacent to the current coding unit may bedirectly referred to and used.

Alternatively, if a current coding unit is predicted based on encodinginformation of adjacent data units, data units adjacent to the currentcoding unit are searched using encoding information of the data units,and the searched adjacent coding units may be referred for predictingthe current coding unit.

FIG. 13 is a diagram for explaining a relationship between a codingunit, a prediction unit or a partition, and a transform unit, accordingto encoding mode information of Table 1.

A maximum coding unit 1300 includes coding units 1302, 1304, 1306, 1312,1314, 1316, and 1318 of coded depths. Here, since the coding unit 1318is a coding unit of a coded depth, split information may be set to 0.Information about a partition type of the coding unit 1318 having a sizeof 2N×2N may be set to be one of a partition type 1322 having a size of2N×2N, a partition type 1324 having a size of 2N×N, a partition type1326 having a size of N×2N, a partition type 1328 having a size of N×N,a partition type 1332 having a size of 2N×nU, a partition type 1334having a size of 2N×nD, a partition type 1336 having a size of nL×2N,and a partition type 1338 having a size of nR×2N.

When the partition type is set to be symmetrical, i.e. the partitiontype 1322, 1324, 1326, or 1328, a transform unit 1342 having a size of2N×2N is set if split information (TU size flag) of a transform unit is0, and a transform unit 1344 having a size of N×N is set if a TU sizeflag is 1.

When the partition type is set to be asymmetrical, i.e., the partitiontype 1332, 1334, 1336, or 1338, a transform unit 1352 having a size of2N×2N is set if a TU size flag is 0, and a transform unit 1354 having asize of N/2×N/2 is set if a TU size flag is 1.

Referring to FIG. 13, the TU size flag is a flag having a value or 0 or1, but the TU size flag is not limited to 1 bit, and a transform unitmay be hierarchically split having a tree structure while the TU sizeflag increases from 0.

In this case, the size of a transform unit that has been actually usedmay be expressed by using a TU size flag of a transform unit, accordingto an exemplary embodiment, together with a maximum size and minimumsize of the transform unit. According to an exemplary embodiment, theapparatus 100 is capable of encoding maximum transform unit sizeinformation, minimum transform unit size information, and a maximum TUsize flag. The result of encoding the maximum transform unit sizeinformation, the minimum transform unit size information, and themaximum TU size flag may be inserted into an SPS. According to anexemplary embodiment, the apparatus 200 may decode video by using themaximum transform unit size information, the minimum transform unit sizeinformation, and the maximum TU size flag.

Intra prediction performed by the intra prediction unit 410 of the videoencoding apparatus 100 illustrated in FIG. 4 and the intra predictionunit 550 of the video decoding apparatus 200 illustrated in FIG. 5 willnow be described in detail. In the following description, an encodingunit denotes a current encoded block in an encoding process of an image,and a decoding unit denotes a current decoded block in a decodingprocess of an image. The encoding unit and the decoding unit aredifferent only in that the encoding unit is used in the encoding processand the decoding unit is used in the decoding. For the consistency ofterms, except for a particular case, the encoding unit and the decodingunit are referred to as a coding unit in both the encoding and decodingprocesses. Also, one of ordinary skill in the art would understand bythe present specification that an intra prediction method and apparatusaccording to an exemplary embodiment may also be applied to performintra prediction in a general video codec.

FIG. 14 illustrates a number of intra prediction modes according to asize of a coding unit, according to an exemplary embodiment.

In FIG. 14, a number of intra prediction modes to be applied to a codingunit (a decoding unit in a decoding step) may vary according to a sizeof the coding unit. For example, referring to FIG. 14, when a size of acoding unit to be intra-predicted is N×N, numbers of intra predictionmodes to be actually performed on coding units having sizes of 2×2, 4×4,16×16, 32×32, 64×64, and 128×128 may be set to be 5, 9, 9, 17, 33, 5,and 5 (in the case of Example 2). For another example, when a size of acoding unit to be intra-predicted is N×N, numbers of intra predictionmodes to be actually performed on coding units having sizes of 2×2, 4×4,8×8, 16×16, 32×32, 64×64, and 128×128 may be set to be 3, 17, 34, 34,34, 5, and 5. The reason why a number of intra prediction modes to beperformed varies according to a size of a coding unit is that overheadfor encoding prediction mode information varies according to the size ofthe coding unit. In other words, in the case of a coding unit having asmall size, although it occupies a small part of an entire image,overhead for transmitting additional information such as a predictionmode of the coding unit having the small size may be high. Accordingly,if the coding unit having the small size is encoded by using too manyprediction modes, the amount of bits may be increased and compressionefficiency may be reduced. A coding unit having a large size, forexample, a coding unit having a size greater than 64×64, is oftenselected as a coding unit for a flat area. If the coding unit having thelarge size is encoded by using too many prediction modes, however,compression efficiency may also be reduced.

Accordingly, in FIG. 14, if sizes of coding units are roughly classifiedinto at least three sizes N1×N1 (2≦N1≦4, N1 is an integer), N2×N2(8≦N2≦32, N2 is an integer), and N3×N3 (64≦N3, N3 is an integer), anumber of intra prediction modes to be performed on the coding unithaving the size of N1×N1 is A1 (A1 is a positive integer), a number ofintra prediction modes to be performed on the coding unit having thesize of N2×N2 is A2 (A2 is a positive integer), and a number of intraprediction modes to be performed on the coding unit having the size ofN3×N3 is A3 (A3 is a positive integer), a number of intra predictionmodes to be performed according to a size of each coding unit may be setto satisfy a relationship of A3≦A1≦A2. That is, when a current pictureis roughly divided into a coding unit having a small size, a coding unithaving an intermediate size, and a coding unit having a large size, thecoding unit having the intermediate size may be set to have a greatestnumber of prediction modes, and the coding unit having the small sizeand the coding unit having the large size may be set to have arelatively smaller number of prediction modes. However, the presentexemplary embodiment is not limited thereto, and the coding unit havingthe small size and the coding unit having the large size may be set tohave a great number of prediction modes. A number of prediction modesvarying according to a size of each coding unit illustrated in FIG. 14is an example, and may be changed.

FIG. 15A is a diagram for explaining an intra prediction mode applied toa coding unit having a predetermined size, according to an exemplaryembodiment.

Referring to FIGS. 14 and 15A, for example, when intra prediction isperformed on a coding unit having a size of 4×4, the coding unit havingthe size of 4×4 may have a vertical mode (mode 0), a horizontal mode(mode 1), a direct current (DC) mode (mode 2), a diagonal down left mode(mode 3), a diagonal down right mode (mode 4), a vertical right mode(mode 5), a horizontal down mode (mode 6), a vertical left mode (mode7), and a horizontal up mode (mode 8).

FIG. 15B illustrates directions of the intra prediction modes of FIG.15A. In FIG. 15B, a numeral shown at an end of an arrow denotes acorresponding mode value when prediction is performed in a directionmarked by the arrow. Here, the mode 2 is a DC prediction mode with nodirectivity and thus is not shown.

FIG. 15C illustrates an intra prediction method performed on the codingunit of FIG. 15A.

Referring to FIG. 15C, a prediction coding unit is generated by usingneighboring pixels A-M of a current coding unit in an available intraprediction mode determined by a size of a coding unit. For example, anoperation of prediction-encoding a current coding unit having a size of4×4 in the mode 0, that is, the vertical mode, will now be explained.First, pixel values of pixels A through D adjacent above the currentcoding unit having the size of 4×4 are predicted to be pixel values ofthe current coding unit having the size of 4×4. That is, a pixel valueof the pixel A is predicted to be pixel values of four pixels of a firstcolumn of the current coding unit having the size of 4×4, a value of thepixel B is predicted to be pixel values of four pixels of a secondcolumn of the current coding unit having the size of 4×4, a value of thepixel C is predicted to be pixel values of four pixels of a third columnof the current coding unit having the size of 4×4, and a value of thepixel D is predicted to be pixel values of four pixels of a fourthcolumn of the current coding unit having the size of 4×4. Next, an errorvalue between actual pixel values of pixels included in the original 4×4current coding unit and pixel values of pixels included in the 4×4current coding unit predicted using the pixels A through D is obtainedand encoded.

FIG. 16 is a diagram for explaining an intra prediction mode applied toa coding unit having a predetermined size, according to anotherexemplary embodiment.

Referring to FIGS. 14 and 16, when intra prediction is performed on acoding unit having a size of 2×2, the coding unit having the size of 2×2may have 5 modes, that is, a vertical mode, a horizontal mode, a DCmode, a plane mode, and a diagonal down right mode.

If a coding unit having a size of 32×32 has 33 intra prediction modes asshown in FIG. 14, it is necessary to set directions of the 33 intraprediction modes. In order to set intra prediction modes having variousdirections other than the intra prediction modes illustrated in FIGS. 15and 16, a prediction direction for selecting a neighboring pixel to beused as a reference pixel about a pixel in a coding unit is set by usingdx and dy parameters. For example, when each of the 33 prediction modesis represented as a mode N (N is an integer from 0 to 32), a mode 0 maybe set to be a vertical mode, a mode 1 is set to be a horizontal mode, amode 2 is set to be a DC mode, and a mode 3 is set to be a plane mode,and each of a mode 4 through a mode 31 may be set to be a predictionmode having a directivity of tan⁻¹(dy/dx) by using (dx, dy) representedas one of (1,−1), (1,1), (1,2), (2,1), (1,−2), (2,1), (1,−2), (2,−1),(2,−11), (5,−7), (10,−7), (11,3), (4,3), (1,11), (1,−1), (12,−3),(1,−11), (1,−7), (3,−10), (5,−6), (7,−6), (7,−4), (11,1), (6,1), (8,3),(5,3), (5,7), (2,7), (5,−7), and (4,−3) as shown in Table 2.

TABLE 2 mode # dx dy mode # dx Dy mode 4 1 −1 mode 18 1 −11 mode 5 1 1mode 19 1 −7 mode 6 1 2 mode 20 3 −10 mode 7 2 1 mode 21 5 −6 mode 8 1−2 mode 22 7 −6 mode 9 2 −1 mode 23 7 −4 mode 10 2 −11 mode 24 11 1 mode11 5 −7 mode 25 6 1 mode 12 10 −7 mode 26 8 3 mode 13 11 3 mode 27 5 3mode 14 4 3 mode 28 5 7 mode 15 1 11 mode 29 2 7 mode 16 1 −1 mode 30 5−7 mode 17 12 −3 mode 31 4 −3 The mode 0 is a vertical mode, the mode 1is a horizontal mode, the mode 2 is a DC mode, the mode 3 is a planemode, and the mode 32 is a bilinear mode.

A last mode 32 may be set to be a bilinear mode using bilinearinterpolation as will be described with reference to FIG. 18.

FIGS. 17A through 17C are reference diagrams for explaining intraprediction modes having various directivities, according to an exemplaryembodiment.

As described above with reference to Table 2, intra prediction modes mayhave various directivities of tan⁻¹(dy/dx) by using a plurality of (dx,dy) parameters.

Referring to FIG. 17A, neighboring pixels A and B located on an extendedline 150 having an angle of tan⁻¹(dy/dx) determined according to (dx,dy) of each mode shown in Table 2 about a current pixel P to bepredicted in a current coding unit may be used as a predictor for thecurrent pixel P. In this case, neighboring pixels used as a predictormay be pixels of previous coding units at up, left, right up, and leftdown sides of a current coding unit, which are previously encoded andrestored. Also, if the extended line 150 passes between, not through,neighboring pixels of integer locations, neighboring pixels close to theextended line 150 may be used as a predictor. For example, a neighboringpixel closest to the extended line 150 may be used as a predictor. Also,an average value between neighboring pixels closer to the extended line150 from among the neighboring pixels may be used as a predictor, or aweighted average value considering a distance between an intersection ofthe extended line 150 and neighboring pixels close to the extended line150 may be used as a predictor for the current pixel P. Also, it may besignaled which neighboring pixel between neighboring pixel on X-axis andneighboring pixel Y-axis, like neighboring pixels A and B, is used as apredictor for the current pixel P in every prediction unit.

FIGS. 17B and 17C are reference diagrams for explaining a process ofgenerating a predictor when the extended line 150 of FIG. 17A passesbetween, not through, neighboring pixels of integer locations.

Referring to FIG. 17B, if the extended line 150 having an angle oftan⁻¹(dy/dx) that is determined according to (dx, dy) of each modepasses between a neighboring pixel A 151 and a neighboring pixel B 152of integer locations, a weighted average value considering a distancebetween an intersection of the extended line 150 and the neighboringpixels A 151 and B 152 close to the extended line 150 may be used as apredictor as described above. For example, if a distance between theneighboring pixel A 151 and the intersection of the extended line 150having the angle of tan⁻¹(dy/dx) is f, and a distance between theneighboring pixel B 152 and the intersection of the extended line 150 isg, a predictor for the current pixel P may be obtained as(A*g+B*f)/(f+g). Here, f and g may be each a normalized distance usingan integer. If software or hardware is used, the predictor for thecurrent pixel P may be obtained by shift operation as (g*A+f*B+2)>>2. Asshown in FIG. 17B, if the extended line 150 passes through a firstquarter close to the neighboring pixel A 151 from among four partsobtained by quartering a distance between the neighboring pixel A 151and the neighboring pixel B 152 of the integer locations, the predictorfor the current pixel P may be acquired as (3*A+B)/4. Such operation maybe performed by shift operation considering rounding-off to a nearestinteger like (3*A+B+2)>>2.

Meanwhile, if the extended line 150 having the angle of tan⁻¹(dy/dx)that is determined according to (dx, dy) of each mode passes between theneighboring pixel A 151 and the neighboring pixel B 152 of the integerlocations, a section between the neighboring pixel A 151 and theneighboring pixel B 152 may be divided into a predetermined number ofareas, and a weighted average value considering a distance between anintersection and the neighboring pixel A 151 and the neighboring pixel B152 in each divided area may be used as a prediction value. For example,referring to FIG. 17C, a section between the neighboring pixel A 151 andthe neighboring pixel B 152 may be divided into five sections P1 throughP5 as shown in FIG. 17C, a representative weighted average valueconsidering a distance between an intersection and the neighboring pixelA 151 and the neighboring pixel B 152 in each section may be determined,and the representative weighted average value may be used as a predictorfor the current pixel P. In detail, if the extended line 150 passesthrough the section P1, a value of the neighboring pixel A may bedetermined as a predictor for the current pixel P. If the extended line150 passes through the section P2, a weighted average value(3*A+1*B+2)>>2 considering a distance between the neighboring pixels Aand B and a middle point of the section P2 may be determined as apredictor for the current pixel P. If the extended line 150 passesthrough the section P3, a weighted average value (2*A+2*B+2)>>2considering a distance between the neighboring pixels A and B and amiddle point of the section P3 may be determined as a predictor for thecurrent pixel P. If the extended line 150 passes through the section P4,a weighted average value (1*A+3*B+2)>>2 considering a distance betweenthe neighboring pixels A and B and a middle point of the section P4 maybe determined as a predictor for the current pixel P. If the extendedline 150 passes through the section P5, a value of the neighboring pixelB may be determined as a predictor for the current pixel P.

Also, if two neighboring pixels, that is, the neighboring pixel A on theup side and the neighboring pixel B on the left side meet the extendedline 150 as shown in FIG. 17A, an average value of the neighboring pixelA and the neighboring pixel B may be used as a predictor for the currentpixel P, or if (dx*dy) is a positive value, the neighboring pixel A onthe up side may be used, and if (dx*dy) is a negative value, theneighboring pixel B on the left side may be used.

The intra prediction modes having various directivities as shown inTable 2 may be preset at an encoding end and a decoding end, and only acorresponding index of an intra prediction mode set for each coding unitmay be transmitted.

FIG. 29 is a diagram illustrating a current coding unit 2900 andneighboring pixels 2910 and 2920 to be used for intra prediction,according to an exemplary embodiment.

Referring to FIG. 29, neighboring pixels 2910 at the upper side of thecurrent coding unit 2900 and neighboring pixels 2920 at the left side ofthe current coding unit 2900 may be used for intra prediction of thecurrent coding unit 2900. As shown in FIG. 29, like lower portion ofneighboring pixels 2920, downleft pixels included in the neighboringblock not yet encoded can be used also for intra prediction of currentcoding unit 2900. The number of the neighboring pixels 2910 and 2920used for intra prediction of the current coding unit 2900 is not limitedthereto and may be changed in consideration of the directivity of anintra prediction mode applied to the current coding unit 2900. Thus,according to an exemplary embodiment, the neighboring pixels used forintra prediction of the current coding unit may include left belowneighboring pixels and right above neighboring pixel about currentcoding unit as well as left neighboring pixels and above neighboringpixels.

FIG. 18 is a reference diagram for explaining a bilinear mode accordingto an exemplary embodiment.

Referring to FIG. 18, in a bilinear mode, a geometric average valueconsidering distances to up, down, left, and right borders of thecurrent pixel P and pixels located at the up, down, left, and rightborders about the current pixel P to be predicted in a current codingunit is calculated and a result of the calculation is used as apredictor for the current pixel P. That is, in a bilinear mode, ageometric average value of a distances to up, down, left, and rightborders of the current pixel P and a pixel A 161, a pixel B 162, a pixelD 166, and a pixel E 167 which are located at the up, down, left, andright borders of the current pixel P is used as a predictor for thecurrent pixel P. Since the bilinear mode is one of intra predictionmodes, neighboring pixels on up and left sides which are previouslyencoded and then restored should also be used as reference pixels duringprediction. Accordingly, corresponding pixel values in the currentcoding unit are not used as the pixel A 161 and the pixel B, but virtualpixel values generated by using neighboring pixels on up and left sidesare used.

For example, first, a virtual pixel C 163 at a right down side of acurrent coding unit may be calculated by using an average value of aneighboring pixel LeftDownPixel 165 at a left down side and aneighboring pixel RightUpPixel 164 at a right up side adjacent to thecurrent coding unit as shown in Equation 1.C=0.5(LeftDownPixel+RightUpPixel)  [Equation 1]

The virtual pixel C 163 may be obtained by shifting operation as TheEquation 1 may be the predictor for the current pixel P may be obtainedby shift operation as C=0.5(LeftDownPixel+RightUpPixel+1)>>1.

Next, a value of a virtual pixel A 161 located on the down border whenthe current pixel P is extended downward by considering a distance W2 tothe right border and a distance W1 to the left border of the currentpixel P may be obtained by using an average value of the neighboringpixel LeftDownPixel 165 and the virtual pixel C 163 considering thedistance W1 and W2. For example, the value of the virtual pixel A 161may be calculated using one equation shown in Equation 2.A=(C*W1+LeftDownPixel*W2)/(W1+W2);A=(C*W1+LeftDownPixel*W2+((W1+W2)/2))/(W1+W2)  [Equation 2]

When a value of W1+W2 in Equation 2 is a power of 2, like 2^n,A=(C*W1+LeftDownPixel*W2+((W1+W2)/2))/(W1+W2) may be calculated by shiftoperation as A=(C*W1+LeftDownPixel*W2+2^(n−1))>>n without division.

Likewise, a value of a virtual pixel B 162 located on the right borderwhen the current pixel P is extended rightward by considering a distanceh2 to the down border and a distance h1 to the up border of the currentpixel P may be obtained by using an average value of the neighboringpixel RightUpPixel 164 considering the distance h1 and h2. For example,the value of the virtual pixel B 162 may be calculated using oneequation shown in Equation 3.B=(C*h1+RightUpPixel*h2)/(h1+h2);B=(C*h1+RightUpPixel*h2+((h1+h2)/2))/(h1+h2)  [Equation 3]

When a value of h1+h2 in Equation 3 is a power of 2, like 2^m,B=(C*h1+RightUpPixel*h2+((h1+h2)/2))/(h1+h2) may be calculated by shiftoperation as B=(C*h1+RightUpPixel*h2+2^(m−1))>>m without division.

Once the values of the virtual pixel B 162 on the right border and thevirtual pixel A 161 on the down border of the current pixel P 160 aredetermined by using Equations 1 through 3, a predictor for the currentpixel P 160 may be determined by using an average value of A+B+D+E. Indetail, a weighted average value considering a distance between thecurrent pixel P 160 and the virtual pixel A 161, the virtual pixel B162, the pixel D 166, and the pixel E 167 or an average value of A+B+D+Emay be used as a predictor for the current pixel P 160. For example, ifa weighted average value is used and the size of block is 16×16, apredictor for the current pixel P may be obtained as(h1*A+h2*D+W1*B+W2*E+16)>>5. Such bilinear prediction is applied to allpixels in the current coding unit, and a prediction coding unit of thecurrent coding unit in a bilinear prediction mode is generated.

Since prediction encoding is performed according to intra predictionmodes that vary according to a size of a coding unit, more efficientcompression may be achieved according to characteristics of an image.

Since a greater number of intra prediction modes than intra predictionmodes used in a conventional codec are used according to a size of acoding unit according to an exemplary embodiment, compatibility with theconventional codec may become a problem. In a conventional art, 9 intraprediction modes at the most may be used as shown in FIGS. 13A and 13B.Accordingly, it is necessary to map intra prediction modes havingvarious directions selected according to an exemplary embodiment to oneof a smaller number of intra prediction modes. That is, when a number ofavailable intra prediction modes of a current coding unit is N1 (N1 isan integer), in order to make the available intra prediction modes ofthe current coding unit compatible with a coding unit of a predeterminedsize including N2 (N2 is an integer different from N1) intra predictionmodes, the intra prediction modes of the current coding unit may bemapped to an intra prediction mode having a most similar direction fromamong the N2 intra prediction modes. For example, a total of 33 intraprediction modes are available as shown in Table 2 in the current codingunit, and it is assumed that an intra prediction mode finally applied tothe current coding unit is the mode 14, that is, (dx,dy)=(4,3), having adirectivity of tan⁻¹(¾)≈36.87 (degrees). In this case, in order to matchthe intra prediction mode applied to the current block to one of 9 intraprediction modes as shown in FIGS. 15A and 15B, the mode 4 (down_right)mode having a most similar directivity to the directivity of 36.87(degrees) may be selected. That is, the mode 14 of Table 2 may be mappedto the mode 4 shown in FIG. 15A. Likewise, if an intra prediction modeapplied to the current coding unit is selected to be the mode 15, thatis, (dx,dy)=(1,11), from among the 33 available intra prediction modesof Table 2, since a directivity of the intra prediction mode applied tothe current coding unit is tan⁻¹(11)≈84.80 (degrees), the mode 0(vertical) of FIG. 13 having a most similar directivity to thedirectivity 84.80 (degrees) may be mapped to the mode 15.

Meanwhile, in order to decode a coding unit encoded through intraprediction, prediction mode information about through which intraprediction mode a current coding unit is encoded is required.Accordingly, when an image is encoded, information about an intraprediction mode of the current coding unit is added to a bitstream, andat this time, if the information about the intra prediction mode isadded as it is to the bitstream for each coding unit, overhead isincreased, thereby reducing compression efficiency. Accordingly, theinformation about the intra prediction mode of the current coding unitdetermined as a result of encoding of the current coding unit is nottransmitted as it is, but only a difference value between a value of anactual intra prediction mode and a prediction value of an intraprediction mode predicted from neighboring coding units is transmitted.

If intra prediction modes having various directions selected accordingto an exemplary embodiment are used, a number of available intraprediction modes may vary according to a size of a coding unit.Accordingly, in order to predict an intra prediction mode of a currentcoding unit, it is necessary to map intra prediction modes ofneighboring coding units to representative intra prediction modes. Here,representative intra prediction modes may be a smaller number of intraprediction modes from among intra prediction modes of availableneighboring coding units, or 9 intra prediction modes as shown in FIG.19.

FIG. 19 is a diagram for explaining a process of generating a predictionvalue of an intra prediction mode of a current coding unit, according toan exemplary embodiment.

Referring to FIG. 19, when the current coding unit is A 170, an intraprediction mode of the current coding unit A 170 may be predicted fromintra prediction modes determined from neighboring coding units. Forexample, if an intra prediction mode determined from a left coding unitB 171 of the current coding unit A 170 is a mode 3 and an intraprediction mode of an up encoding unit C 172 is a mode 4, an intraprediction mode of the current coding unit A 170 may be predicted to bea mode 3 having a smaller value from among the prediction modes of theup coding unit C 172 and the left coding unit B 171. If an intraprediction mode determined as a result of actual intra predictionencoding performed on the current coding unit A 170 is a mode 4, only adifference 1 from the mode 3 that is a value of the intra predictionmode predicted from the neighboring coding units is transmitted as intraprediction mode information for the current coding unit A 170. When animage is decoded, in the same manner, a prediction value of an intraprediction mode of a current decoding unit is generated, a modedifference value transmitted through a bitstream is added to theprediction value of the intra prediction mode, and intra prediction modeinformation actually applied to the current decoding unit is obtained.Although only the neighboring coding units located on the up and leftsides of the current coding unit are used in the above description, anintra prediction mode of the current coding unit A 170 may be predictedby using other neighboring coding units such as E and D of FIG. 19.

Since intra prediction modes actually performed vary according to sizesof coding units, an intra prediction mode predicted from neighboringcoding units may not be matched with an intra prediction mode of thecurrent coding unit. Accordingly, in order to predict an intraprediction mode of a current coding unit from neighboring coding unitshaving different sizes, a mapping process for unifying intra predictionmodes of the coding units having different intra prediction modes isrequired.

FIGS. 20A and 20B are reference diagrams for explaining a mappingprocess for unifying intra prediction modes of coding units havingdifferent sizes, according to an exemplary embodiment.

Referring to FIG. 20A, it is assumed that a current coding unit A 180has a size of 16×16, a left coding unit B 181 has a size of 8×8, and anup coding unit C 182 has a size of 4×4. Also, as shown in FIG. 14, it isassumed that numbers of available intra prediction modes of the codingunits having the sizes of 4×4, 8×8, and 16×16 are respectively 9, 9, and33. In this case, since the numbers of the available intra predictionmodes of the left coding unit B 181 and the up coding unit C 182 aredifferent from the number of the available intra prediction modes of thecurrent coding unit A 180, an intra prediction mode predicted from theleft coding unit B 181 and the up coding unit C 182 is not suitable tobe used as a prediction value of an intra prediction mode of the currentcoding unit A 180. Accordingly, in FIG. 20A, intra prediction modes ofthe neighboring coding units B and C 181 and 182 are respectivelychanged to first and second representative intra prediction modes havinga most similar direction from among a predetermined number ofrepresentative intra prediction modes, and a mode with a smaller valuefrom among the first and second representative intra prediction modes isselected as a final representative intra prediction mode. And, an intraprediction mode having a most similar direction to the representativeintra prediction mode selected from among the intra prediction modesavailable according to a size of the current coding unit 1080 ispredicted to be an intra prediction mode of the current coding unit1080.

Alternatively, referring to FIG. 20B, it is assumed that a currentcoding unit A has a size of 16×16, a left coding unit B has a size of32×32, and an up coding unit C has a size of 8×8. Also, as shown in FIG.14, it is assumed that numbers of available intra prediction modes ofthe coding units having the sizes of 8×8, 16×16, and 32×32 arerespectively 9, 9, and 33. Also, it is assumed that an intra predictionmode of the left coding unit B is a mode 4, and an intra prediction modeof the up coding unit C is a mode 31. In this case, since the intraprediction modes of the left coding unit B and the up coding unit C arenot compatible with each other, each of the intra prediction modes ofthe left coding unit B and the up coding unit C is mapped to one ofrepresentative intra prediction modes shown in FIG. 21. Since the mode31 that is the intra prediction mode of the left coding unit B has adirectivity of (dx,dy)=(4, −3) as shown in Table 2, a mode 5 having amost similar directivity to tan⁻¹(−¾) from among the representativeintra prediction modes of FIG. 21 is mapped, and since the intraprediction mode mode 4 of the up coding unit C has the same directivityas that of the mode 4 from among the representative intra predictionmodes of FIG. 21, the mode 4 is mapped.

The mode 4 having a smaller mode value from among the mode 5 that is themapped intra prediction mode of the left coding unit B and the mode 4that is the mapped intra prediction mode of the up coding unit C may bedetermined to be a prediction value of an intra prediction mode of thecurrent coding unit, and only a mode difference value between an actualintra prediction mode and a predicted intra prediction mode of thecurrent coding unit may be encoded as prediction mode information of thecurrent coding unit.

FIG. 21 is a reference diagram for explaining a process of mapping intraprediction modes of neighboring coding units to one of representativeintra prediction modes, according to an exemplary embodiment. In FIG.21, as representative intra prediction modes, a vertical mode, ahorizontal mode, a DC mode, a diagonal left mode, a diagonal right mode,a vertical right mode, a vertical left mode, a horizontal up mode, and ahorizontal down mode are set. However, the representative intraprediction modes are not limited thereto, and may be set to have avarious number of directivities.

Referring to FIG. 21, a predetermined number of representative intraprediction modes are previously set, and intra prediction modes ofneighboring coding units are mapped to a representative intra predictionmode having a most similar direction. For example, if a determined intraprediction mode of the up coding unit A is an intra prediction modeMODE_A(190) having a directivity, the intra prediction mode MODE_A(190)of the up coding unit A is mapped to MODE 1 having a most similardirection from among 9 preset representative intra prediction modes 1through 9. Likewise, if a determined intra prediction mode of the leftcoding unit B is an intra prediction mode MOD_B(191) having adirectivity, the intra prediction mode MODE_B(191) of the left codingunit B is mapped to MODE 5 having a most similar direction from amongthe 9 preset representative intra prediction modes 1 through 9.

Next, a mode having a smaller value from among a first representativeintra prediction mode and a second representative intra prediction modeis selected as a final representative intra prediction mode of aneighboring coding unit. The reason why a representative intraprediction mode having a smaller mode value is selected is that asmaller mode value is set to more often generated intra predictionmodes. That is, if different intra prediction modes are predicted fromneighboring coding units, since an intra prediction mode having asmaller mode value has a higher occurrence possibility, it is preferableto select a prediction mode having a smaller value as a predictor for aprediction mode of the current coding unit when there are differentprediction modes.

Although a representative intra prediction mode is selected fromneighboring coding units, the representative intra prediction mode maynot be used as it is as a predictor of an intra prediction mode of acurrent coding unit sometimes. If the current coding unit A 180 has 33intra prediction modes and a representative intra prediction mode has 9intra prediction modes as described with reference to FIG. 20, an intraprediction mode of the current coding unit A 180 corresponding to arepresentative intra prediction mode does not exist. In this case, in asimilar manner to that used to map intra prediction modes of neighboringcoding units to a representative intra prediction mode as describedabove, an intra prediction mode having a most similar direction to arepresentative intra prediction mode selected from intra predictionmodes according to a size of a current coding unit may be selected as afinal predictor of an intra prediction mode of the current coding unit.For example, if a representative intra prediction mode finally selectedfrom neighboring coding units in FIG. 21 is MODE 1, an intra predictionmode having a most similar directivity to MODE 1 from among intraprediction modes available according to the size of the current codingunit may be finally selected as a predictor of the intra prediction modeof the current coding unit.

Meanwhile, as described with reference to FIGS. 15A through 15C, if apredictor for the current pixel P is generated by using neighboringpixels on or close to the extended line 150, the extended line 150 hasactually a directivity of tan⁻¹(dy/dx). In order to calculate thedirectivity, since division (dy/dx) is necessary, calculation is madedown to decimal places when hardware or software is used, therebyincreasing the amount of calculation. Accordingly, a process of settingdx and dy is used in order to reduce the amount of calculation when aprediction direction for selecting neighboring pixels to be used asreference pixels about a pixel in a coding unit is set by using dx, anddy parameters in a similar manner to that described with reference toTable 2.

FIG. 25 is a diagram for explaining a relationship between a currentpixel and neighboring pixels located on an extended line having adirectivity of (dy/dx), according to an exemplary embodiment.

Referring to FIG. 25, it is assumed that a location of the current pixelP is P(j,i), and an up neighboring pixel and a left neighboring pixel Blocated on an extended line 2510 having a directivity, that is, agradient, of tan⁻¹(dy/dx) and passing through the current pixel P arerespectively A and B. When it is assumed that locations of upneighboring pixels correspond to an X-axis on a coordinate plane, andlocations of left neighboring pixels correspond to a y-axis on thecoordinate plate, the up neighboring pixel A is located at(j+i*dx/dy,0), and the left neighboring pixel B is located at(0,i+j*dy/dx). Accordingly, in order to determine any one of the upneighboring pixel A and the left neighboring pixel B for predicting thecurrent pixel P, division, such as dx/dy or dy/dx, is required. Suchdivision is very complex as described above, thereby reducing acalculation speed of software or hardware.

Accordingly, a value of any one of dx and dy representing a directivityof a prediction mode for determining neighboring pixels may bedetermined to be a power of 2. That is, when n and m are integers, dxand dy may be 2^n and 2^m, respectively.

Referring to FIG. 25, if the left neighboring pixel B is used as apredictor for the current pixel P and dx has a value of 2^n, j*dy/dxnecessary to determine (0,i+j*dy/dx) that is a location of the leftneighboring pixel B becomes (j*dy/(2^n)), and division using such apower of 2 is easily obtained through shift operation as (j*dy)>>n,thereby reducing the amount of calculation.

Likewise, if the up neighboring pixel A is used as a predictor for thecurrent pixel P and dy has a value of 2^m, i*dx/dy necessary todetermine (j+i*dx/dy,0) that is a location of the up neighboring pixel Abecomes (i*dx)/(2^m), and division using such a power of 2 is easilyobtained through shift operation as (i*dx)>>m.

FIG. 26 is a diagram for explaining a change in a neighboring pixellocated on an extended line having a directivity of (dx,dy) according toa location of a current pixel, according to an exemplary embodiment.

As a neighboring pixel necessary for prediction according to a locationof a current pixel, any one of an up neighboring pixel and a leftneighboring pixel is selected.

Referring to FIG. 26, when a current pixel 2610 is P(j,i) and ispredicted by using a neighboring pixel located in a predictiondirection, an up pixel A is used to predict the current pixel P 2610.When the current pixel 2610 is Q(b,a), a left pixel B is used to predictthe current pixel Q 2620.

If only a dy component of a y-axis direction from among (dx, dy)representing a prediction direction has a power of 2 like 2^m, while theup pixel A in FIG. 24 may be determined through shift operation withoutdivision such as (j+(i*dx)>>m, 0), the left pixel B requires divisionsuch as (0, a+b*2^m/dx). Accordingly, in order to exclude division whena predictor is generated for all pixels of a current block, all of dxand dy may have a type of power of 2.

FIGS. 27 and 28 are diagrams for explaining a method of determining anintra prediction mode direction, according to exemplary embodiments.

In general, there are many cases where linear patterns shown in an imageor a video signal are vertical or horizontal. Accordingly, when intraprediction modes having various directivities are defined by usingparameters dx and dy, image coding efficiency may be improved bydefining values dx and dy as follows.

In detail, if dy has a fixed value of 2^m, an absolute value of dx maybe set so that a distance between prediction directions close to avertical direction is narrow, and a distance between prediction modescloser to a horizontal direction is wider. For example, referring toFIG. 27, if dy has a value of 2^4, that is, 16, a value of dx may be setto be 1,2,3,4,6,9,12, 16,0,−1,−2,−3,−4,−6,−9,−12, and −16 so that adistance between prediction directions close to a vertical direction isnarrow and a distance between prediction modes closer to a horizontaldirection is wider.

Likewise, if dx has a fixed value of 2^n, an absolute value of dy may beset so that a distance between prediction directions close to ahorizontal direction is narrow and a distance between prediction modescloser to a vertical direction is wider. For example, referring to FIG.28, if dx has a value of 2^4, that is, 16, a value of dy may be set tobe 1,2,3,4,6,9,12, 16,0,−1,−2,−3,−4,−6,−9,−12, and −16 so that adistance between prediction directions close to a horizontal directionis narrow and a distance between prediction modes closer to a verticaldirection is wider.

Also, when one of values of dx and dy is fixed, the remaining value maybe set to be increased according to a prediction mode. For example, ifdy is fixed, a distance between dx may be set to be increased by apredetermined value. Also, an angle of a horizontal direction and avertical direction may be divided in predetermined units, and such anincreased amount may be set in each of the divided angles. For example,if dy is fixed, a value of dx may be set to have an increased amount ofa in a section less than 15 degrees, an increased amount of b in asection between 15 degrees and 30 degrees, and an increased width of cin a section greater than 30 degrees. In this case, in order to havesuch a shape as shown in FIG. 25, the value of dx may be set to satisfya relationship of a≦b≦c.

For example, prediction modes described with reference to FIGS. 25through 28 may be defined as a prediction mode having a directivity oftan⁻¹(dy/dx) by using (dx, dy) as shown in Tables 3 through 5.

TABLE 3 dx Dy dx dy dx dy −32 32 21 32 32 13 −26 32 26 32 32 17 −21 3232 32 32 21 −17 32 32 −26 32 26 −13 32 32 −21 32 32 −9 32 32 −17 −5 3232 −13 −2 32 32 −9 0 32 32 −5 2 32 32 −2 5 32 32 0 9 32 32 2 13 32 32 517 32 32 9

TABLE 4 dx Dy dx dy dx Dy −32 32 19 32 32 10 −25 32 25 32 32 14 9 32 3232 32 19 −14 32 32 −25 32 25 −10 32 32 −19 32 32 −6 32 32 −14 −3 32 32−10 −1 32 32 −6 0 32 32 −3 1 32 32 −1 3 32 32 0 6 32 32 1 10 32 32 3 1432 32 6

TABLE 5 dx Dy dx dy dx dy −32 32 23 32 32 15 −27 32 27 32 32 19 −23 3232 32 32 23 −19 32 32 −27 32 27 −15 32 32 −23 32 32 −11 32 32 −19 −7 3232 −15 −3 32 32 −11 0 32 32 −7 3 32 32 −3 7 32 32 0 11 32 32 3 15 32 327 19 32 32 11

FIG. 22 is a block diagram of an intra prediction apparatus 200according to an exemplary embodiment. The intra prediction apparatus 200may operate as the intra predictor 410 of the apparatus 400 of FIG. 4and the intra predictor 550 of the apparatus 500 of FIG. 5.

Referring to FIG. 22, an intra prediction mode determiner 2010determines an intra prediction mode to be applied to a current codingunit according to a size of each of coding units split based on amaximum coding unit and depth as described above. That is, the intraprediction mode determiner 2010 determines intra prediction modes to beapplied according to a size of a current coding unit from among intraprediction modes having various directions.

An intra prediction performer 2020 performs intra prediction on eachcoding unit by using the determined intra prediction modes. An optimumintra prediction mode having a least error value from among error valuesbetween an original coding unit and a prediction coding unit generatedas a result of the intra prediction performed by the intra predictionperformer 2020 is determined as a final intra prediction mode of thecoding unit.

Meanwhile, if the intra prediction apparatus 2000 illustrated in FIG. 22is applied to a decoding apparatus, the intra prediction mode determiner2010 determines a size of a current decoding unit by using a maximumcoding unit extracted from a bitstream encoded by the entropy decoder520 of FIG. 5 and depth information that is obtained by hierarchicallysplit the maximum coding unit. Also, the intra prediction performer 2020generates a prediction decoding unit by performing intra prediction on adecoding unit according to an extracted intra prediction mode. Theprediction decoding unit is added to residual data restored from thebitstream to perform decoding on the decoding unit.

FIG. 23 is a flowchart illustrating a method of encoding an image,according to an exemplary embodiment.

Referring to FIG. 23, in operation 2110, a current picture is dividedinto at least one block. As described above, the current picture may bedivided based on a maximum coding unit that is a coding unit having amaximum size and a depth that is obtained by hierarchically split themaximum coding unit.

In operation 2120, an intra prediction mode to be performed for acurrent block according to a size of the current block is determined. Asdescribed above, the intra prediction mode includes a prediction modefor performing prediction by using pixels of neighboring blocks on orclose to an extended line having an angle of tan⁻¹(dy/dx) about eachpixel inside the current block.

In operation 2130, intra prediction is performed on the current blockaccording to the determined intra prediction mode. An intra predictionmode having a least prediction error value from among intra predictionmodes is selected as a final intra prediction mode of the current block.

FIG. 24 is a flowchart illustrating a method of decoding an image,according to an exemplary embodiment.

Referring to FIG. 24, in operation 2210, a current picture is dividedinto at least one block having a predetermined size. Here, the currentpicture may be divided based on a maximum decoding unit that is adecoding unit having a maximum size extracted from a bitstream and depthinformation that is obtained by hierarchically split the maximumdecoding unit.

In operation 2220, information about an intra prediction mode applied toa current block is extracted from the bitstream. The intra predictionmode includes a prediction mode for performing prediction by usingpixels of neighboring blocks on or close to an extended line having anangle of tan⁻¹(dy/dx) (dx and dy are integers) about each pixel insidethe current block. As described above with reference to FIGS. 19 through21, if a predictor of an intra prediction mode predicted from intraprediction modes of the neighboring decoding units is used, intraprediction modes of the neighboring decoding units of a current decodingunit are mapped to representative intra prediction modes, and arepresentative intra prediction mode having a smaller mode value fromamong the representative intra prediction modes is selected as a finalrepresentative intra prediction mode. And, an intra prediction modehaving a most similar direction to the final representative intraprediction mode from among available intra prediction modes determinedaccording to a size of the current decoding unit is selected as apredictor of the intra prediction mode of the current decoding unit, adifference value between predictors of the intra prediction mode and anactual intra prediction mode included in the bitstream is extracted, andthe difference value is added to the predictor of the intra predictionmode, thereby determining the intra prediction mode of the currentdecoding unit.

In operation 2230, a decoding unit is decoded by performing intraprediction on the decoding unit according to the extracted intraprediction mode.

The exemplary embodiments may be written as computer programs and can beimplemented in general-use digital computers that execute the programsusing a computer readable recording medium. Examples of the computerreadable recording medium include magnetic storage media (e.g., ROM,floppy disks, hard disks, etc.) and optical recording media (e.g.,CD-ROMs, or DVDs).

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the invention as defined by the appended claims. The preferredembodiments should be considered in descriptive sense only and not forpurposes of limitation. Therefore, the scope of the invention is definednot by the detailed description of the invention but by the appendedclaims, and all differences within the scope will be construed as beingincluded in the present invention.

What is claimed is:
 1. A method of decoding an image, the methodcomprising: extracting information about an intra prediction mode of acurrent block to be decoded, from a bitstream, the intra prediction modeindicating a particular direction among a plurality of directions, theparticular direction being indicated by one of dx number in a horizontaldirection and a fixed number in a vertical direction, and dy number inthe vertical direction and a fixed number in the horizontal direction;and performing intra prediction on the current block according to theintra prediction mode, the intra prediction comprising: determining anumber of neighboring pixels located on one side among a left side ofthe current block and an upper side of the current block according to aposition of a current pixel and the one of dx number in the horizontaldirection and the dy number in the vertical direction, where the dx anddy are integers according to the particular direction; determining oneof a location of a neighboring pixel located on the left side of thecurrent block determined based on the dy number in the verticaldirection and the fixed number in the horizontal direction and alocation of a neighboring pixel located on the upper side of the currentblock determined based on the dx number in the horizontal direction andthe fixed number in the vertical direction; when the number of theneighboring pixels is 1, obtaining a prediction value of the currentpixel based on the neighboring pixel; and when the number of theneighboring pixels is 2, obtaining the prediction value of the currentpixel based on a weighted average of the neighboring pixels, wherein thedx number and the dy number are determined among {32, 26, 21, 17, 13, 9,5, 2, 0, −2, −5, −9, −13, −17, −21, −26} according to the intraprediction mode of the current block, wherein the fixed integar numberis power of 2, and wherein: the image is split into a plurality ofmaximum coding units, according to information about a maximum size of acoding unit, a maximum coding unit, from among the plurality of maximumcoding units, is hierarchically split into one or more coding units ofdepths according to split information, a coding unit of a current depthis one of rectangular data units split from a coding unit of an upperdepth, when the split information indicates a split for the currentdepth, the coding unit of the current depth is split into coding unitsof a lower depth, independently from neighboring coding units, when thesplit information indicates a non-split of the current depth, one ormore prediction units are obtained from the coding unit of the currentdepth, and the weighted average is determined based on one of the dxnumber and the dy number, and the location of one or more neighboringpixels.
 2. The method of claim 1, wherein the intra prediction modecomprises a prediction mode performing prediction using N, N beinginteger, pixels of neighboring blocks previously decoded and restored,located on or close to an extended line.
 3. The method of claim 2,wherein the N pixels of neighboring blocks are pixels of a neighboringblock adjacent to the current block or not encoded block.
 4. The methodof claim 1, wherein, when sizes of coding units are roughly classifiedinto at least three sizes N1×N1 (N1 is an integer), N2×N2 (N2 is aninteger), and N3×N3 (N3 is an integer and N1<N2<N3), a number of intraprediction modes to be performed on the coding unit having the size ofN2×N2 is greater than numbers of intra prediction modes to be performedon the coding units having the size of N1×N1 and N3×N3.
 5. The method ofclaim 1, wherein numbers of intra prediction modes to be performed oncoding units having sizes of 4×4, 8×8, 16×16, 32×32, 64×64, and 128×128are 17, 34, 34, 34, 5, and 5, respectively.
 6. An apparatus for encodingan image, the apparatus comprising: an intra prediction mode determinerwhich determines an intra prediction mode of a current block to beencoded, the intra prediction mode indicating a particular directionamong a plurality of directions, the particular direction beingindicated by one of dx number in a horizontal direction and a fixednumber in a vertical direction, and dy number in the vertical directionand a fixed number in the horizontal direction; and an intra predictionperformer which performs intra prediction on the current block to beencoded by determining a number of neighboring pixels located on oneside among a left side of the current block and an upper side of thecurrent block according to a position of a current pixel and the one ofthe dx number in the horizontal direction and the dy number in thevertical direction, where dx and dy are integers according to theparticular direction, determining one of a location of a neighboringpixel located on the left side of the current block determined based onthe dy number in the vertical direction and the fixed number in thehorizontal direction and a location of a neighboring pixel located onthe upper side of the current block determined based on the dx number inthe horizontal direction and the fixed number in the vertical direction,when the number of the neighboring pixels is 1, obtaining a predictionvalue of the current pixel based on the neighboring pixel, and when thenumber of the neighboring pixels is 2, obtaining the prediction value ofthe current pixel based on a weighted average of the neighboring pixels,wherein the dx number and the dy number are determined among {32, 26,21, 17, 13, 9, 5, 2, 0, −2, −5, −9, −13, −17, −21, −26 } according tothe intra prediction mode of the current block, wherein the fixedinteger number is power of 2 and wherein: the image is split into aplurality of maximum coding units, according to information about amaximum size of a coding unit, the maximum coding unit is hierarchicallysplit into one or more coding units of depths according to splitinformation, a coding unit of a current depth is one of rectangular dataunits split from a coding unit of an upper depth, when the splitinformation indicates a split for the current depth, the coding unit ofthe current depth is split into coding units of a lower depth,independently from neighboring coding units, when the split informationindicates a non-split of the current depth, one or more prediction unitsare obtained from the coding unit of the current depth, and the weightedaverage is determined based on one of the dx number and the dy number,and the location of one or more neighboring pixels.